Nuclear magnetic resonance (NMR) techniques have been used to measure blood flow noninvasively. Volumetric flow signals have been obtained from many regions of the body using both cylindrical and flat crossed coil detectors. The cylindrical detector as measured flow individually in the radial and ulnar arteries and in the brachial artery. Flat crossed coil detectors have been used to detect volumetric flow signals in the popliteal, posterior tibial, carotid and middle cerebral arteries, the region of the heart and the descending aorta. A newly developed "focusing" technique has expanded the capability of the NMR flowmeter by erasing all signals except those in a limited region to allow flow measurement from individual arteries with the exclusion of others. While these signals have been obtained, additional sensitivity is required our routine clinical application. The aim of this research is to improve the sensitivity of the NMR flowmeter for measurement of blood flow in deeper vessels in the brain, and from smaller arteries and veins. This will be accomplished by an in-depth biomedical engineering study of the factors that determine signal-to-noise ratio of the NMR blood flow signal. A study of the sources of detector noise and means of reducing this component will be conducted. Electronic evaluation, in vitro flow-loop testing and in vivo studies with human volunteers will be used to evaluate the detector sensitivity throughout this program. The NMR flowmeter is a useful clinical tool for measurement of blood flow in major vessels. This research program is aimed at expanding its capabilities and its range of clinical application. The NMR blood flowmeter is unique, since it does not require contact with the subject, and may be used where there are casts, bandages, as well as over regions of trauma or injury. It can measure volumetric flow quantitatively and on-line. Improvement in signal-to-noise ratio will enhance the accuracy and repeatability of such on-line measurements.